High resolution mass spectrometry is widely used in the detection and identification of molecular structures and the study of chemical and physical processes. A variety of different techniques are known for the generation of a mass spectrum using various trapping and detection methods.
One such technique is Fourier Transform Ion Cyclotron Resonance (FT-ICR). FT-ICR uses the principle of a Cyclotron, wherein a high frequency voltage excites ions to move in spiral orbits within an ICR measurement cell. The ions in the cell orbit as coherent bunches along the same radial paths but at different frequencies. The frequency of the circular motion (the Cyclotron frequency) is proportional to the ion mass. A set of detector electrodes are provided and an image current is induced in these by the coherent orbiting ions. The amplitude and frequency of the detected signal are indicative of the quantity and mass of the ions. A mass spectrum is obtainable by carrying out a Fourier Transform of the “transient”, that is, the signal produced at the detector's electrodes.
An attraction of FT-ICR is its ultrahigh resolution (up to 1,000,000 in certain circumstances, and typically well in excess of 100,000). However, relative to other known mass spectrometry techniques, such as Time Of Flight Mass Spectrometry (TOF-MS), or 3-D (Paul type) traps, FT-ICR Mass Spectrometry (hereinafter referred to as FTMS) provides particular challenges if a meaningful mass spectrum is to be obtained, particularly at a high resolution. For example, as detailed in our co-pending patent application number GB0305420.2, it is important to optimise various system parameters.
Compared with other methods of mass spectrometry, FTMS allows a relatively narrow range of mass to charge (m/z) ratios to be captured in the measurement cell during any particular scan. Partly, this is a result of the need to place the cell within the bore of a superconducting magnet. A further difficulty is caused by the manner of injection of ions into the measurement cell. Ions are supplied to the measurement cell from an external source. Electrostatic injection to the cell, or the use of a multipole injection arrangement (see U.S. Pat. No. 4,535,235) result in a time of flight spread in the ions as they pass from the previous, ion storage stage, into the FTMS measurement cell. Although the techniques described in the above referenced GB0305420.2 help to minimise this time of flight spread, some spreading is inevitable and this means that the lighter, faster ions arrive at the cell sometime before the heavier, slower ions. As a consequence, if the cell is opened and closed shortly after the ions are ejected from the previous stage ion storage, ions of smaller m/z tend to be captured. If the cell is left open for a longer period, to attempt to capture slower ions having a higher m/z, then the lighter ions that have arrived at the cell tend to be lost.
It would accordingly be desirable for a method and apparatus to be provided which would allow a wider range mass spectrum to be generated in FTMS.